Recently, it is required strongly to improve engine exhaust gas emission characteristics in USA, Europe, Japan and the like with tightening of emission control of automobile internal combustion engines. As high performance of a catalyst and high accuracy of a catalyst control are improved, the emission characteristic of the engine exhaust gas discharged at engine starting has become most important. Improvement of air-fuel ratio control accuracy is effective to improve the emission characteristic at engine starting. Deterioration of the emission characteristic is brought due to the following causes. For example, in an inlet port-fuel injection type engine (hereinafter referred to as Multi-Point Injection), a part or almost of fuel injected at engine starting does not flow into a cylinder (combustion chamber) because it adheres to a wall of an intake passage of the engine. Then, as time goes by, since the adhering fuel becomes fuel flow on a wall surface of the intake passage and gradually flows into the combustion chamber and results in degradation of the air-fuel ratio, the deterioration of the exhaust emission characteristic is caused. Incidentally, even in in-cylinders injection type engine, a part or almost of fuel injected at engine starting remains in the cylinder (inner wall surface of the combustion chamber, piston surface or the like) or a crank case. After that, as time goes by, since the remaining fuel burns at next and subsequent times in the combustion chamber, deterioration of the exhaust emission characteristic is caused due to the remaining fuel. Under these circumstances, for example, as shown in Japanese laid-open patent publication No. 2001-34287 and Japanese laid open patent publication No. Hei8-261037, a technique is required to control the combustion air-fuel ratio just after engine starting considering influences of the adhering fuel on the wall flow caused in the intake passage at engine starting.
The engine control system described in the patent document 1, discloses that the quantity of injection fuel is corrected by reducing by a predetermined quantity in the second cycle and subsequent times in accordance with the number of combustion cycles after engine starting. Since this control system corrects an injection fuel quantity by reducing the injection fuel quantity by a regularly predetermined quantity, and accordingly, when the quantity of the adhered fuel flowing on a wall surface of the intake passage etc. in the first cycle changes, the air-fuel ratio control accuracy deteriorates. Accordingly, the exhaust emission characteristic deteriorates. Additionally, when the operation condition of the engine changes, for example, a filling efficiency changes, deterioration of the exhaust emission characteristic is caused as in the case of the above-mentioned adhered fuel flowing on a wall surface at the engine starting. Namely, there is a problem of a low robustness.
In the engine control system disclosed in Japanese laid open patent publication No. Hei 8-261037, the initial value of increment coefficient (increasing coefficient) after the second engine starting is determined based on the adhered fuel after completion of engine starting and the adhered fuel quantity at completion of engine starting is determined based on the start injection fuel quantity and inflow rate of start injection fuel quantity into cylinders (combustion chamber). This control system obtains the quantity of fuel flowing into a cylinder (combustion chamber) from the inflow rate of injection fuel quantity into cylinders at engine starting. Concretely, this engine control system predicts the quantity of fuel flowing into a cylinder by multiplying the quantity of injection fuel at engine starting by a suction coefficient of the combustion chamber. Therefore, it is not based on the actual suction (burning) fuel quantity, and the inflow coefficient accuracy greatly depends on the temperature estimation accuracy of the inlet valve of the engine.
However, keeping the estimation accuracy is a quite difficult from the viewpoint of information of the current sensor (engine cooling water temperature sensor and intake air temperature), because heat conductivity characteristic to the inlet valve is complicated. Furthermore, a variable valve train system will become general use in the future, and thereby, the fuel inflow rate to the combustion chamber can be changed by changing closing and opening timing of the inlet and outlet valves, and by changing lift quantity of the inlet and outlet valves. Therefore, complicated computation needs to estimate these quantities and an error cause may be increase. Further, as the suction coefficient is determined based on a certain fuel property, if the suction fuel quantity is estimated using the suction coefficient when the fuel property changes, an error occurs. As described above, estimation computation is difficult in corresponding various kinds of the condition changes and its low robust is a problem.